Silicon, specifically polysilicon, is a basic material from which a large variety of semiconductor products are made. Silicon forms the foundation of many integrated circuit technologies, as well as photovoltaic transducers. Of particular industry interest is high purity silicon.
Processes for producing polysilicon may be carried out in different types of reaction devices, including chemical vapor deposition reactors and fluidized bed reactors. Various aspects of the chemical vapor deposition (CVD) process, in particular the Siemens or “hot wire” process, have been described, for example in a variety of U.S. patents or published applications (see, e.g., U.S. Pat. Nos. 3,011,877; 3,099,534; 3,147,141; 4,150,168; 4,179,530; 4,311,545; and 5,118,485).
Silane and trichlorosilane are both used as feed materials for the production of polysilicon. Silane is more readily available as a high purity feedstock because it is easier to purify than trichlorosilane. Production of trichlorosilane introduces boron and phosphorus impurities, which are difficult to remove because they tend to have boiling points that are close to the boiling point of trichlorosilane itself. Although both silane and trichlorosilane are used as feedstock in Siemens-type chemical vapor deposition reactors, trichlorosilane is more commonly used in such reactors. Silane, on the other hand, is a more commonly used feedstock for production of polysilicon in fluidized bed reactors.
Silane has drawbacks when used as a feedstock for either chemical vapor deposition or fluidized bed reactors. Producing polysilicon from silane in a Siemens-type chemical vapor deposition reactor may require up to twice the electrical energy compared to producing polysilicon from trichlorosilane in such a reactor. Further, the capital costs are high because a Siemens-type chemical vapor deposition reactor yields only about half as much polysilicon from silane as from trichlorosilane. Thus, any advantages resulting from higher purity of silane are offset by higher capital and operating costs in producing polysilicon from silane in a Siemens-type chemical vapor deposition reactor. This has led to the common use of trichlorosilane as feed material for production of polysilicon in such reactors.
Silane as feedstock for production of polysilicon in a fluidized bed reactor has advantages regarding electrical energy usage compared to production in Siemens-type chemical vapor deposition reactors. However, there are disadvantages that offset the operating cost advantages. In using the fluidized bed reactor, the process itself may result in a lower quality polysilicon product even though the purity of the feedstock is high. For example, polysilicon produced in a fluidized bed reactor may also include metal impurities from the equipment used in providing the fluidized bed due to the typically abrasive conditions found within a fluidized bed. Further, polysilicon dust may be formed, which may interfere with operation by forming ultra-fine particulate material within the reactor and may also decrease the overall yield. Further, polysilicon produced in a fluidized bed reactor may contain residual hydrogen gas, which must be removed by subsequent processing. Thus, although high purity silane may be available, the use of high purity silane as a feedstock for the production of polysilicon in either type of reactor may be limited by the disadvantages noted.
Chemical vapor deposition reactors may be used to convert a first chemical species, present in vapor or gaseous form, to solid material. The deposition may and commonly does involve the conversion or decomposition of the first chemical species to one or more second chemical species, one of which second chemical species is a substantially non-volatile species.
Decomposition and deposition of the second chemical species on a substrate is induced by heating the substrate to a temperature at which the first chemical species decomposes on contact with the substrate to provide one or more of the aforementioned second chemical species, one of which second chemical species is a substantially non-volatile species. Solids so formed and deposited may be in the form of successive annular layers deposited on bulk forms, such as immobile rods, or deposited on mobile substrates, such as beads, grains, or other similar particulate matter chemically and structurally suitable for use as a substrate.
Beads are currently produced, or grown, in a fluidized bed reactor where an accumulation of dust, comprised of the desired product of the decomposition reaction, acting as seeds for additional growth, and pre-formed beads, also comprised of the desired product of the decomposition reaction, are suspended in a gas stream passing through the fluidized bed reactor. Due to the high gas volumes needed to fluidize the bed within a fluidized bed reactor, where the volume of the gas containing the first chemical species is insufficient to fluidize the bed within the reactor, a supplemental fluidizing gas such as an inert or marginally reactive gas is used to provide the gas volume necessary to fluidize the bed. As an inert or only marginally reactive gas, the ratio of the gas containing the first chemical species to the supplemental fluidizing gas may be used to control or otherwise limit the reaction rate within or the product matrix provided by the fluidized bed reactor.
The use of a supplemental fluidizing gas however can increase the size of process equipment and also increases separation and treatment costs to separate any unreacted or decomposed first chemical species present in the gas exiting the fluidized bed reactor from the supplemental gas used within the fluidized bed reactor.
In a conventional fluidized bed reactor, silane and one or more diluents such as hydrogen are used to fluidize the bed. Since the fluidized bed temperature is maintained at a level sufficient to thermally decompose silane, the gases used to fluidize the bed, due to intimate contact with the bed, are necessarily heated to the same bed temperature. For example, silane gas fed to a fluidized bed reactor operating at a temperature exceeding 500° C. is itself heated to its auto-decomposition temperature. This heating causes some of the silane gas to undergo spontaneous thermal decomposition which creates an extremely fine (e.g., having a particle diameter of 0.1 micron or less) silicon powder that is often referred to as “amorphous dust” or “poly-powder.” Silane forming poly-powder instead of the preferred polysilicon deposition on a substrate represents lost yield and unfavorably impacts production economics. The very fine poly-powder is electrostatic and is fairly difficult to separate from product particles for removal from the system. Additionally, if the poly-powder is not separated, off-specification polysilicon granules (i.e., polysilicon granules having a particle size less than the desired diameter of about 1.5 mm) are formed, further eroding yield and further unfavorably impacting production economics.
In some instances, a silane yield loss to poly-powder is on the order of about 1%, but may range from about 0.5% to about 5%. The average poly-powder particle size is typically about 0.1 micron, but can range from about 0.05 microns to about 1 micron. A 1% yield loss can therefore create around 1×1016 poly-powder particles. Unless these fine poly-powder particles are removed from the fluidized bed, the poly-powder will provide particles having only 1/3,000th of the industry desired diameter of 1.5 mm. Thus the ability to efficiently remove ultra-fine particles from the fluidized bed or from the fluid bed reactor off-gas is important. However, electrostatic forces often hinder filtering the ultra-fine poly-powder from a finished product or fluid bed reactor off-gas. Therefore, processes that minimize or ideally avoid the formation of the ultra-fine poly-powder are quite advantageous.